550 



SUMMARY OF CUKRENT RESEARCHES RELATING TO 



of some objectives.* A large prism, however, gives a correspondingly 

 large pencil of rays, and in the instrument we examined there was a 

 glare of light in the left ocular, which led some one who used it to 

 think there was a defect in the fitting of the prism. On covering the 

 right-hand half of the eye-lens of the left ocular, the glare was 

 entirely stopped out, and the binocular effect was perfect." 



Fig. 91. 



Fase's Portable Dissecting Microscope.— We regret to have 

 omitted to state in describing this instrument at p. 415 that it was 



* There is some confusion liere between the mechanical cutting down of the 

 opening of tlie objective caused by the setting of the prism and the optical cutting 

 down of the aperture-angle, which with the Wenham prism is necessarily one-half, 

 only half of tlie rays from tlie objective being directed into each tube.— [Ed.] 



